1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a duty-cycle type solenoid valve used in a hydraulic apparatus for a motor vehicle, for example.
2. Description of the Prior Art
A known example of apparatus for controlling a duty-cycle type solenoid valve is employed in a hydraulic apparatus for a motor vehicle as disclosed in Japanese laid-open patent publication No. 3-9169.
In the known control apparatus, a control unit for an automatic transmission (hereinafter abbreviated to "A/T") controls the ON/OFF state of a line pressure solenoid valve so as to control a throttle pressure, when a throttle sensor detects a throttle opening (or a degree of depression of an accelerator pedal) and transmits an output voltage signal representing the throttle opening to the A/T control unit. Then, the A/T control unit controls a pressure regulator valve using this throttle pressure as a signal pressure, so as to regulate a working oil pressure delivered by an oil pump to a line pressure whose level is most suitable for the current running condition of the vehicle.
The line pressure solenoid valve is repeatedly placed in its ON and OFF states at a frequency of 50 Hz, for example, thereby to open and close a drain path to control the throttle pressure to a desired level. As this line pressure solenoid valve, a duty-cycle type solenoid valve may be used, in which the ON duration and OFF duration in one cycle can be freely controlled in a range of 0 to 100%.
In the known duty-cycle type solenoid valve (line pressure solenoid valve) as described above, a shut-off valve that is included in the solenoid valve and uses a needle as a valve body moves rapidly to close the drain path, and therefore the oil pressure P in a pipe is rapidly increased to a high level (PS) due to the inertia force of oil in the pipe, at the moment "ts" when the drain path is closed by the valve body, as shown in FIG. 12. As a result, vibrations (oil vibrations) may arise in the oil path due to water hammer phenomena.
Also, when the drain path is closed by the shut-off valve of the solenoid valve, the valve body hits a valve seat, and causes noise due to collision between the valve body and the valve seat, and vibrations at a location where the solenoid valve is installed, which result in reduced durability of the solenoid valve.
Further, since the valve body also moves rapidly to open the solenoid valve, large shocks may occur due to abutting contact between the valve body and a stopper on which the valve body rests when opening the valve.